There is little risk of a breakdown under warranty if you don't change the "lifetime" fluids. Rear end and transmission fluid will not get so bad that it gums up and causes a breakdown, it will just lead to increased wear. Mike Miller even comments that changing the fluids as he recommends doesn't make much difference in the first 100K miles. But your transmission will shift smoother and operate more quietly with good lubrication. I hope to keep my BMW more than 100K miles and fluids are pretty inexpensive compared to the components they protect.